3386 Table I. Cmr Data for 2 and 35
Acknowledgment. We are pleased to acknowledge the support of this work by the National Science Foundation. We also thank Professor H. Giinther for permission to cite unpublished data.
~
(1 8) Gillette Research Foundation Fellow, 1972-1973
Sir : The facility with which cis-bicyclo[6.1.O]nona-2,4,6triene (1) undergoes protonation to yield the parent monocyclic 1,3-bishomotropylium ion was reported in 1971.' At that time, several factors were considered to mitigate against the possibility that the resulting cation was transoid (2b), and its cis conformation (2a) was
favored. More recently, however, chlorosulfonyl isocyanate2 and tetracyanoethylene additions to 1 have been found to occur in a manner which suggests the possible transient involvement of 2b or the structurally related noninteracting cation 2c. It was noted that the double-Mobius nature4 of 2b could possibly allow for homoaromatic delocalization, thereby rationalizing the observed charge distribution in 2. Additionally, kinetically controlled electrophilic attack from the exo direction on the folded (rather than extended) conformation of 1 was established. 2 , 3 A more detailed study of the protonation of 1 and its derivatives appeared highly desirable, and we therefore undertook to assess the behavior of these polyenes in superacids under conditions of long life. The high symmetry of 2 was clearly evident in its off-resonance decoupled cmr spectrum obtained in FS03H-S02CIF The chemical shifts of the consolution at -95". stituent carbon atoms are summarized in Table I where direct comparison with monohomotropylium ion (3) (generated under comparable conditions at - 60") is made. Although the ring current effect in 2 is clearly weaker than in 3,'3 the two spectra are seen to compare favorably. (1) P. Warner and S. Winstein, J . Amer. Chem. Soc., 93, 1284 (1971). (2) L. A. Paquette, M. J. Broadhurst, C. Lee, and J. Clardy, ibid., 94, 630 (1972); L. A. Paquette and M. J. Broadhurst, ibid., 94, 632 (1972). (3) J. Clardy, L. K . Read, M. J. Broadhurst, and L. A . Paquette, ibid., 94, 2904 (1972). ( 4 ) This term is employed to emphasize the unusual three-dimensional orbital construct of 2b and refers simply to the r orbital directionality inherent in this cation. ( 5 ) This diminished interaction is also revealed in carbon shift differences. For example, AaL3C= ~ C -t I 6121 in ion 2 is about $46
Journal of the American Chemical Society
Chem shift.b 6
ct, C6 cz, c4
56.7~ 25.8 10.3 52.~3~ 162.2
CS
c? cs,c9
Stuart W. Staley,* Arnold W. Orvedal18 Department of Chemistry, University of Maryland College Park, Maryland 20742 Received February 20, 1973
Protonation of cis-Bicyclo[6.1.O]nona-2,4,6-trienesin Superacidic Media. Carbon-13 Nuclear Magnetic Resonance Study of the Mono- and 1,3-Bishomotropylium Ions under Long Life Conditions
C atom in 2
c 6 7
--
~~
C atom in 3
Chem shift,b 6
c1, CI
71.5 40.0 48.5 49.0
G, c6
c3, Cb
C4 CS
150.0
a A Varian HA-100 instrument was employed and the Fast Fourier Transform (FFT) method was employed. The carbon shifts are cited relative to external CSZ. c Interchangeable values.
When the deuteration of 1 in FSO3D-SO2C1F at ca.
- 125" was reexamined, distribution of the isotope between the inner and outer positions of C8 or C9 was found to be 50: 50 within our limits of analysis. Therefore, little if any stereoselectivity obtains in the incorporation of deuterium while proceeding from 1 t o 2. syn-9-Methyl derivative 4a, which cannot attain a folded conformation and consequently lacks the ability to achieve good initial overlap of the internal cyclopropane bond with the developing charge center at Cz, gave only broad polymeric-type absorptions upon protonation at -78 to -150". In contrast, anti isomer 4b readily gave an nmr spectrum (Figure 1) comparable to that of 2 which, after decoupling clarified all assignments, was consistent only with 5a or 5b (Table 11).
Table 11. 100-MHz Pmr Data for Protonated anti-9-Methyl-cis- bicyclol6.1 .O]nona-2,4,6-triene (5) (FSO,H-SOZCIF, 1:4 (v/v), ca. -130") ~~
Chem shift,aProton
7
Coupling constants, Hz
H,
0.98 2.05 2.19 2.92
z J3,4 = 6.0 f 0 . 5 = 9.5 f0.5 J4.5 = 9 . 0 i 0 . 5 Js,so = 9 . 5 & 0 . 5 , J 5 , 9 i =
H2
H4 H 6
2.95
H* HI
a
3.02 3.53
Hgo
6.06
Hxi
7.86
Hgi CH3
8.30 8.46
Jz.3
J1.z
11.0 f 0 . 5 = 1 0 . 0 zk 0.5, J & 9 i = ca. 0 Ji.si = 11 .O f 0 . 5 k,?= 8 . 0 & 0.5, J T ,=~ca. ~0 Jgo,gi = 1 2 . 0 i0 . 5 Je,,9o
Chemical shifts measured relative to internal CHDClz
(T
4.70).
The remarkable dichotomy in behavior between 4a and 4b points again to the fact that protonation of this ring system at C3 must necessarily occur from the folded conformation to arrive at a homoaromatic species. 2 , 3 ppm, a much larger value than that for the comparable difference in I 6C4 = 22.5 ppm. Equally revealing is the fact ion 3, A61C = ~ C that the A 6 W values of such homotropylium ions are generally smaller than those in monocyclic pentadienyl cations (54.4-89 ppm).
1 95:lO 1 M a y 16, 1973
3387
In the case of 4b, this process initially places the methyl group on the outside of the ring. For conformational reasons, the initially formed ion must be 5a. Should the transoid species be the thermodynamically stable conformer under these conditions, then 5a is considered to be the correct structure since double bridge inversion t o place methyl in the sterically less favorable inner position does not operate (uide infra). If, on the other hand, the ion is cisoid, the structure must be 5b (inversion of the methylene bridge) rather than the sterically more demanding ion with the methyl inside. It is important to note that 5 is a static structure and that rapid exchange between 5a and 5b is not important. Deuteration of 4b as before led in this case to stereoselective (75 & deuterium incorporation at the 9i site. Numerous attempts to protonate 4c and 4d in various media at low temperatures have proved unsuccessful. Not unexpectedly, 2. the 9,9-dimethyl derivative 4e likewise did not give rise to a bishomotropylium ion. Comparison of the nmr data for the two 1,3-bishomotropylium ions reveals, inter alia, that the average chemical shift of Hsi and H9i (7 8.08) in 5 compares well with that of the corresponding protons in 2 (8.09). The unsymmetrical nature of 5 does, however, reveal for the first time the magnitude of JC,, (8.0 Hz). Although this finding can be readily accommodated by 5a in which significant twisting of the Ce,, bond is necessary to maximize overlap across the methylene bridges, the attendant decrease in C6-C, overlap could so affect J that a clearcut distinction between 5a and 5b is unfortunately not possible on this basis. When consideration is given to the stereochemistry of the deuteration process as revealed by the spectra of the long-lived cations, a greater nef endo positioning of deuterium is seen in 5 . Unfortunately, the stereoselectivity of H+ (or D+) attack cannot be determined unambiguously. Consequently, these observations may be satisfied by one of several options: (a) random deuteration to give trans ions 2b and 5a with no flipping of either bridge atom, (b) stereoselective exo electrophilic attack followed by a single flip of either bridge in 2b to give 2a and by inversion of only the methylene bridge in 5a with formation of 5b, (c) in the unsubstituted example, the first inversion (trans cis), when operative, is followed by a rapid flip of the other bridge (cis + trans). Since the type c sequence in the case of 4b would position methyl in an endo orientation (not reconcilable with the measured coupling constants), operation of this mechanism can be discounted in this instance. Its generality can consequently be questioned. Also, examination of models indicates that the methyl group in 4b is sufficiently remote from the sites (two equivalent positions are available) of initial electrophilic bonding not to cause the sizable selectivity difference noted upon deuteration of 1 and 4b. Therefore, ran( 6 ) The rather large uncertainty quoted for this inneriouter deuterium
ratio arises from several factors. First, partial overlap of the Hgi and methyl absorptions renders integration of this peak difficult. The Hs, multiplet is clearly separated from all other resonances, but the total amount of entering deuterium is unknown (the DSOaF has 2-3% proton contamination). As a consequence of an unknown isotope effect, the amount of H at Ce could vary from 1.0 to 1.1. I t follows then that if the Hs, peak comprises 7 5 Z of 1.0 hydrogen, the ratio is 75 : 2 5 , but if 1 .I hydrogens are present the ratio now becomes 83 : 17.
P
I1
"*
- '-'
,*70
Figure 1. "r spectrum of protonated 4b.
dom deuteration is seemingly not a suitable mechanistic hypothesis. Thus, the protonation of cis-bicyclo[6.1.O]nonatrienes under conditions of long life would appear to involve initial formation of a trans cation which subsequently experiences conformational inversion of a (less substituted) methylene bridge. Molecular models show that the trans ion is reasonably rigid, fairly strain free, and certainly less demanding than a comparable 1,4bishomotropylium ion.7 Nevertheless, although the transoid intermediates are capable of interception under presumed kinetic control conditions, * , they transmute to their cis counterparts when thermodynamic control can operate. This can be interpreted to mean that 2b (or 2c) resides closer to the transition state for bridge inversion than does 2a. A number of important questions still remain to be answered about the electronic nature of the trans ion8 and about the possible differences which may arise in solvent systems other than superacids. (7) H. A. Corver and R. F. Childs, J . Amer. Chem. Soc., 94, 6201 (1972). (8) Preliminary calculations suggest that 2a and 2b may be nearly isoenergetic. However, the lack of strong dependence on molecular geometry requires highly refined calculations before this point may be assessed accurately (J. Clardy, personal communication). (9) Holder of a NATO Postdoctoral Fellowship (1970-1972) administered by the Science Research Council. (1 0) Considered as publication CXLIII on Stable Carbocations.
Leo A. Paquette,* Michael J. Broadhurstg Department of Chemistry, The Ohio State University Columbus, Ohio 43210 Philip Warner* Department of Chemistry, Iowa State Unifiersity Ames, Iowa 50010 George A. Olah,* lo Gao Liang Department of Chemistry, Case Western Reserve Uniuersity Cleoeland, Ohio 44106 Received September 21, 1972
Stable Carbocations. CLV.' The Ethylenechloronium and Methylchlorocarbenium Ions Sir:
Alkenehalonium ions have long been suggested as intermediates in electrophilic halogen addition and (1) Stable Carbocations. J . Org. Chem., in press.
CLIV: G. A . Olah and Y. K . Mo,
Communications to the Editor
